Stream Design

SSE relay for a design job's progress (ownership-guarded).

Clone of stream_calibration: text/event-stream, Cache-Control no-cache, X-Accel-Buffering no; OWNERSHIP FIRST — a foreign or missing job id returns 404 resource_missing (no enumeration signal). Then subscribe f"design_events:{id}", emit an initial queued snapshot, keepalive ~30s, unwrap the worker's {"object": {...}} envelope, break on terminal status, finally unsubscribe + aclose. Distinct design.updated SSE event name so a stale calibration client cannot read design frames.